The invention relates to a device for high-resolution measurement, in particular for high-resolution absolute measurement of magnetic fields.
The measuring principle is based on the physical effect of macroscopic quantum interference as it occurs in closed circuits made from superconducting materials which are coupled to one another by Josephson tunnel junctions or, in general, what are termed weak links.
It is known to be possible simply to use closed superconducting circuits which contain Josephson junctions or weak links to measure very small magnetic field changes down to the range of fT (10xe2x88x9215 tesla). In the case of devices corresponding to the prior art, what are termed xe2x80x9cSQUIDsxe2x80x9d (Superconducting Quantum Interference Devices), use is simply made of closed superconducting current loops which usually contain two Josephson junctions, but also more in individual applications. If these current loops are driven by a current which is below a critical current, no voltage drops across the junctions. In modern SQUIDs, the current loops are, however, driven by a temporally constant supercritical current, such that a temporally quickly changing AC voltage drops across the two superconducting electrodes on both sides of the junctions. The frequency of this AC voltage depends on the strength of the driving current I0 and the strength of the magnetic flux "PHgr"=Bxe2x8axa5F which penetrates the loop, Bxe2x8axa5 denoting the component, perpendicular to the surface F of the SQUID, of the vector magnetic field {right arrow over (B)}. Serving as easily accessible measured quantity is the DC voltage according to  less than V({right arrow over (B)};I0) greater than , dropping across the current loop, which is produced by time averaging of the quickly changing AC voltage over one or more periods. The calibration curve  less than V({right arrow over (B)};I0) greater than  of such a typical two-DC junctions SQUID is sketched in FIG. 12. Whenever the flux "PHgr" penetrating the loop corresponds to an integral multiple of the elementary flux quantum             Φ      0        =                  h                  2          ⁢          e                    ≅              2        xc3x97                  10                      -            15                          ⁢                  Tm          2                      ,
the calibration curve assumes a minimum, whereas it assumes a maximum for half-integer multiples of the elementary flux quantum "PHgr"0. The calibration curves of all previously known SQUID systems have such a periodicity. For a known area F of the current loop, the component, perpendicular to this area, of the magnetic induction {right arrow over (B)} can be determined up to an integral multiple of "PHgr"0, that is to say it is therefore possible in principle to measure only "PHgr"mod"PHgr"0. Because of the periodicity of the calibration curve  less than V({right arrow over (B)};I0) greater than , it is therefore impossible to use conventional SQUIDs for absolute quantitative precision measurement of the magnetic induction {right arrow over (B)}. At present, this requires the very complicated and expensive combination of this with other physical measurement methods such as, for example, connection to optically pumped magnetometers. The commercial fields of application of SQUIDs are therefore limited to the detection of spatial or temporal, relative field changes such as occur, for example, in the case of material testing or when investigating metabolic processes in biological organisms. However, it is necessary in the case of these applications as well for the order of magnitude of the field changes to be known from the start if the measurement is to permit more than purely qualitative statements or rough estimates.
It is the object of the invention to create a simple device which permits highly precise absolute measurement of, in particular, even time-variant magnetic fields, and in the process can have recourse in full measure to the cryotechnology developed for conventional SQUIDS.
The invention proceeds from a device for high-resolution measurement, in particular for high-resolution absolute measurement of magnetic, in particular time-variant, magnetic fields, which comprises a network of transitions between superconductors which exhibit Josephson effects, called junctions below, the network having closed meshes, denoted by cells below, which in each case have at least two junctions, which junctions are connected in a superconducting fashion, and at least three of these cells being electrically connected in a superconducting and/or nonsuperconducting fashion. The core of the invention resides in the fact that the junctions of the at least three cells can be energized in such a way that a time-variant voltage drops in each case across at least two junctions of a cell, the time average of which voltage does not vanish, and in that the at least three cells are configured differently geometrically in such a way that the magnetic fluxes enclosed by the cells in the case of an existing magnetic field differ from one another in such a way that the frequency spectrum of the voltage response function has no significant "PHgr"0-periodic component with reference to the magnetic flux, or in that, if a discrete frequency spectrum exists, the contribution of the "PHgr"0-periodic component of the discrete frequency spectrum is not dominant by comparison with the non-"PHgr"0-periodic component of the discrete frequency spectrum.
With regard to the periodicity of the voltage response function, it is also possible to select the following functional approach: that the at least three cells are configured differently geometrically in such a way that the magnetic fluxes enclosed by the cells in the case of an existing magnetic field form a ratio to one another in such a way that the period of the voltage response function of the network with reference to the magnetic flux penetrating the network cells in their entirety is greater or very much greater than the value of an elementary flux quantum and/or the voltage response no longer has a "PHgr"0-periodic component. The invention is based on the finding that in the ideal case the voltage response function no longer has a period when the magnetic fluxes enclosed by the cells are not in a rational ratio to one another. In addition, the differences in area between the individual cells are preferably relatively large. In particular, cells connected in a superconducting fashion are superimposed in such a way that the voltage response function no longer has a period.
Consequently, according to the invention different cells are connected to one another specifically, and this is something the person skilled in the art would always want to avoid with conventional SQUID arrangements. This is expressed, for example, in the publication by HANSEN, BINSLEV J., LINDELOF P. E.: Static and dynamic interactions between Josephson junctions. In: Reviews of Modern Physics, Vol. 56, No. 3, July 1984, p. 431 to 459. On page 434, left-hand column, last paragraph and subsequently in the right-hand column, this publication favors a system with identical cells and identical junctions and, by contrast, classifies asymmetries as counterproductive for the functioning of the SQUID described in this regard.
Devices according to the invention (denoted below as superconducting quantum interference filters or SQIFs), by contrast, exhibit the physical effect of multiple macroscopic quantum interference in such a way that the ambiguity of the calibration curves of conventional SQUID magnetometers and SQUID gradiometers is removed.
In a superconducting quantum interference filter, the quantum mechanical wave functions which describe the state of the superconducting solid interfere in such a way that a unique microscopic calibration curve  less than V({right arrow over (B)};I0) greater than  is produced. In the ideal case, the calibration curve  less than V({right arrow over (B)};I0) greater than  of the superconducting quantum interference filter has no periodicity with the period "PHgr"0 and is a function, rising monotonically in a specific measuring range, of the absolute value of the external magnetic field {right arrow over (B)} at the location of the SQIF.
The uniqueness of the calibration curve, and the high sensitivity of superconducting quantum interference filters permit the direct measurement of time-variant electromagnetic fields in a continuous frequency range whose lower bound is at xcexdext≈0 and whose upper bound is currently at several hundred GHz to THz, depending on the type of Josephson junctions or weak links used. This entire frequency range is accessible with the aid of a single, appropriately designed superconducting quantum interference filter. In the detection of electromagnetic waves, the superconducting quantum interference filter operates simultaneously as a receiving antenna, filter and powerful amplifier. The inherent noise of suitably designed quantum interference filters can in this case be smaller by several orders of magnitude than the inherent noise of conventional SQUID magnetometers. A further advantage by comparison with conventional antennas and filters resides in this case in that, inter alia, depending on the measuring principle the frequency range is not a function of the spatial extent of the superconducting quantum interference filter. The spatial extent can influence only the sensitivity.
The production of superconducting quantum interference filters can be performed using known, cost-effective technical methods such as are applied, for example, in modern production of conventional SQUIDs. Since the spatial extent of superconducting quantum interference filters need not differ substantially from the spatial extent of conventional SQUID systems, the cryotechnologies developed for conventional SQUID systems can be taken over directly. There is no need for specific developments in the field of cryotechnology.
In a system made from the cells described above, it is preferable to provide for at least one cell, favorably for the largest part of the cells, exactly two junctions per cell which are connected in a superconducting fashion and connected electrically in parallel. The effects just described can be achieved comparatively simply and effectively by exactly two junctions.
However, the desired effects can also be achieved in a favorable way when more than two junctions are provided in a cell which are connected in a superconducting fashion and connected electrically in parallel, specifically in the form of a series circuit of junctions which is connected in parallel to an individual junction, or in the form of two parallel-connected series circuits of junctions.
However, the effects according to the invention can also be achieved by structures of at least one cell of a network, in the case of which, in addition to a basic form of at least two junctions across which a time-variant voltage whose time average does not vanish drops, in particular in addition to a basic form of two junctions connected electrically in parallel, a further junction or a plurality of further junctions are provided, which junctions are not directly energized (compare FIGS. 2b, 2e and 2f), and therefore there is no voltage drop on average across these junctions. In this case, the connections of all the junctions in the individual cells continues [sic] to be superconducting. Such embodiments can be advantageous, since the screening currents induced in the individual cells by a magnetic field can be reduced by additional junctions. The influence of self-inductances and mutual inductances can thereby be reduced.
The following references can be specified from the literature in relation to the prior art.
A. Barone and G. Paterno, Physics and Applications of the Josephson Effect, John Wiley, 1982.
J. Hinken, Superconducting Electronics, Springer, 1988.
K. K. Likharev, Dynamics of Josephson junctions and circuits. Gordon and Breach, New York, 1991.
T. P. Orlando and K. A. Delin, Foundations of Applied Superconductivity. Addison-Wesley, 1991.
R. D. Parmentier and N. F. Pedersen, Nonlinear superconducting devices and high-Tc materials. World Scientific, 1995.
C. P. Poole, H. A. Farach, and R. J. Creswick, Superconductivity. Academic Press, 1995.
J. B. Ketterson and S.N. Song, Superconductivity. Cambridge University Press, 1995.
S. T. Ruggiero and D. A. Rudman, Superconducting Devices. Academic Press, 1990.
J. C. Gallop et. al. [sic], SQUIDS, the Josephson Effect and Superconducting Electronics. Hilger, 1991.
T. VanDuzer and C. W. Turner, Principles of Superconductive Devices and Circuits. Elsevier, 1981.
J. Oppenlxc3xa4nder, W. Gxc3xcttinger, T. Traeuble, M. Keck, T. Doderer, and R. P. Heubener, IEEE Trans. Supercon. 9, 4337 (1999).
J. Oppenlxc3xa4nder, Ch. Hxc3xa4ussler, and N. Schopohl, J. Appl. Phys. 86, 5775 (1999).
H. Weinstock (editor), SQUID Sensors: Fundamentals, Fabrication and Applications. Kluwer Academic Publishers, 1996.
In a particularly preferred embodiment of the invention, a plurality of cells form a network or a network section, in which all junctions are connected electrically in parallel such that the junctions can be energized in the same direction. In particular, particularly high sensitivities for the measurement of a magnetic field can be achieved by means of such an arrangement when, in this connection, the cells are interconnected in a superconducting fashion.
A plurality of cells or network sections can, however, also be advantageously connected electrically in series such that the junctions in the network can in turn be energized in the same direction. The magnitude of the measurement signal can be increased by this measure, since the voltages at the junctions add together in the series circuit. A particularly high sensitivity can also be achieved by the parallel connection of series arrangements of the plurality of cells or network sections. Since, because of the larger number of cells in such embodiments, the inherent noise is, moreover, sharply reduced, this also permits the detection of magnetic fields whose strength is smaller by several orders of magnitude than in the case of conventional SQUID systems. In this embodiment, the network sections or cells are preferably connected in a superconducting fashion, in particular by means of superconducting twisted-pair cables. The resolution of superconducting quantum interference filters can in this case reach down to the range of aT (10xe2x88x9218 tesla) and below. The calibration curve also remains unique for such measuring ranges, thus rendering possible absolute quantitative measurements of extremely small fields.
The network can be used in a voltage-driven or current-driven fashion.
In order to achieve Josephson effects which are as ideal as possible, it is proposed, furthermore, that the junctions are designed as point contacts.
In order to increase the sensitivity of a device according to the invention, it is further proposed that the geometry of the cell arrangement be designed so as to reduce magnetic crosstalk from one cell to an adjacent cell on the basis of the magnetic self-field produced by a current flowing in the cells.
In a further advantageous refinement of the invention, the network is fitted with a superconducting loop arrangement and/or planar arrangement, which displaces the magnetic field and/or strengthens it in such a way that the magnetic flux produced by a primary magnetic field in these superconducting regions is coupled into the cells of the network. In the case of SQUIDs, this is a known method for increasing their sensitivity since, owing to the xe2x80x9cMeissner effectxe2x80x9d of the superconductor, such loop arrangements have the property of forcing the magnetic flux penetrating them into their outer surroundings. If a SQIF or SQUID is arranged in these outer surroundings, a greatly increased magnetic field then prevails owing to this pickup loop. This holds not only for loop arrangements, but also for planar superconducting regions (xe2x80x9cwashersxe2x80x9d). The term xe2x80x9cflux focusingxe2x80x9d is also used in this context. It has emerged that a SQIF can be coupled very much more effectively to a pickup loop than a conventional SQUID. The point is that, with conventional SQUIDs having a pickup loop, there is the problem that, because of the very different effective area of the SQUID (regularly at most approximately 50 xcexcmxc3x9750 xcexcm) and the effective area of the pickup look (regularly of the order of magnitude of cmxc3x97cm), a very severe mismatching of impedance occurs which constitutes a grave problem particularly with RF applications. Since the effective (total) area of SQIFs is generally very much greater than that of SQUIDs, the problem of a mismatching of impedance is very much less, or is solved, for SQIFs.
In a further preferred refinement of the invention, the cells of the network and/or network sections are spatially aligned, in particular in two-dimensional or three-dimensional space. These measures render it possible to determine individual magnetic field components in addition to the absolute value of the magnetic field. The direction of the magnetic field can therefore be measured in the case of a three-dimensional arrangement in space.
It is further preferred if the current driving the junctions is fed into, and/or led off again from, the network by ohmic resistors which are designed, in particular, as busbar resistors. The point is that measurements have shown that the performance of the SQIF can be substantially improved by feeding the driving current through ohmic resistors.
In a refinement of the invention which is also preferred, individual cells and/or network sections and/or the entire network are/is fitted with a compensation circuit for producing a secondary magnetic field in such a way that the magnetic flux produced by a primary magnetic field can be compensated in a controlled fashion by means of individual cells, network sections or the entire network. This can be realized, in particular, by virtue of the fact that a controllable static or time-variant magnetic field is produced at the location of individual cells and/or network sections and/or the entire network. The measuring range of the superconducting quantum interference filter can thereby be selected arbitrarily in principle.
The device according to the invention is preferably connected to an electronic computer in order, for example, to evaluate the voltage response of the network or to control the compensation circuit.